installing the "GNOME Desktop" group will install GDM (the GNOME display manager), to use it instead of KDM, as root:

systemctl enable --force gdm.service

Note that you can just leave KDE installed and select GNOME at the login screen, and have both DE's installed side-by-side. This is useful if say you want to use some KDE apps under GNOME, or GNOME has gone south and you need a fallback to be able to log in and try to fix the problem... etc.

The cleanest way to get a "pure" GNOME install would be to download the GNOME live media and do a clean install.

You can try to remove most of kde and qt packages but you'll still have some lib packages lying around, and rooting them out one by one is too tedious; this should remove a good number of the kde/qt packages:

yum remove *qt* kdelibs*
if you decide to run this command you'd better run `yum groups install "GNOME Desktop"` again so that any packages that may get removed by `yum remove *qt* kdelibs*` are installed again.